Assembla will let you run SVN, GIT and P4 repo's but what isn't clear is how and more importantly where you do this.

Log into your account on Assembla.com
Go to your project page
Click on the Admin tab
Under the tools section click 'More'
Finally in here you can Add your P4 repro

Now we have a P4 tab in the top menu which has 'Instructions' link underneath that is populated with your user settings, host and port information which is awesome.

Once you have this set up you need to set your depot to stream

'p4 client -S //depot/main -o | p4 client -i'

Edit//
If you are considering putting everything under an initial revision then you might save yourself some pain and remember that some files in the config folder (particularly the ones that check that you've updated your UnrealScript) need to be writable.
C:\UDK\UDK-2013-09\UDKGame\Config
So once you've submitted revision 1 you'll need to check them back about again.